battery age - as batteries age, they have less life.
Look at the projected life: LP-E6 is a 2600mAH charge life. Canon says you get about 1hr, 20 min continuous shooting with that battery which means the camera would pull roughly 2A at full power so more mAH will extend the life using, for example, the battery grip, but turning off the camera between shots and such will help.
Use OEM batteries - I used to get the cheap ones until they started failing on me by running solid for 10 minutes, then suddenly dropping to 0. Never had that happen on the Canon ones, though.
Keep your batteries charged - give em a full charge the night before the shoot. Rechargeable batteries sort of "leak" power over time.
Lower temps effect life, too: colder temps slow down the battery and heat can kill them, too.
Ah, I may have been wrong on the overall life. That "mAH" (milli-Amp-Hours) represents the anticipated life of the battery in milliAmps. Basically, the higher that number, the more life you get from the battery. I gave you numbers based on Canon's estimates in the 70D manual only I thought it was a 2600mAH battery not 1800mAH which means there are suitable Canon replacement batteries that may give you more expected life (maybe up to 30% more?). Since there are so many factors involved with battery life, it's really hard to say for sure (ie: temperature, manufacturer, age of the battery, overall camera use, etc.).